The middle phase microemulsion of Sodium Dodecylsulfonate / 2-Phenyloxyethanol / heptane / Brine was studied at optimal salinity. The fact that structure nonhomogeneity develops upon standing was revealed by cryo-TEM, polarizing microscopy and small angle X-ray diffraction. The structural complexity in this middle phase microemulsion is demonstrated and the cause of structure transformation from spherical micelles to thread-like ones, then to vesicles and to lamellar phase was analyzed using a concentration gradient of surfactant and cosurfactant in the middle phase microemulsion. This experiment also shows the advantages of this new cryo-TEM method.